As market demands increase for display systems showing high quality images, micromirrors and micromirror arrays have blossomed in display applications.
A common parameter in the evaluation and characterization of display systems is contrast. Contrast is defined as the ratio between the brightest and dimmest intensity that a display is capable of rendering in an image. Hence, a display that shows parts of an image as bright white while other areas of the same image are dark black has more contrast than a display which shows the same image in intermediate gray tones.
Creating a display with high contrast is challenging from an engineering standpoint since human perception of brightness is nonlinear. Humans resolve brightness differences between dim objects more precisely than between bright objects. Hence a faithful display must be able to reproduce more finely spaced brightness levels for the display of dim images than for bright ones.
In state-of-the-art display systems incorporating micromirrors, contrast is achieved through digital modulation. The light modulator is a micromechanical structure that incorporates thousands to millions of movable, micromechanical mirrors. Each micromirror rests in one of two possible positions which result in a tiny area of the displayed image appearing white or black. In a given static micromirror state the displayed image is composed of tiny light and dark areas known as pixels. Each pixel is a spot of light at either the maximum or minimum intensity that the display is capable of generating.
Display of intermediate intensity or gray light levels is achieved in conventional devices by alternating a micromirror's position between its white or black position so fast that the flicker between the bright and dark state is not noticeable by a human observer. For example, a micromirror that is made to alternate between its white or black position several hundred or thousands of times per second will project a spot that appears gray to an observer.
Since mirrors in conventional devices have two possible positions (“white” and “black”; “bright” and “dark”; “on” and “off”; “1” or “0”; etc.), gray levels are displayed by sending binary data to each micromirror. For example, a binary sequence in which the display is “on” half the time, e.g. white-black-white-black-white-black-white-black (or, equivalently, “10101010”), results in the display of a spot (equivalently, “image element”, “pixel”, etc.) whose intensity appears to a human observer to be continuously that of a spot of approximate intensity “½” or approximately midway between white and black.
Similarly, the sequence “00000101” or any other sequence with six zeros and two ones would correspond to continuous intensity “¼” or one quarter of the maximum display brightness. In general the fractional brightness observed corresponds to the fraction of time that a given pixel is in its “on” state.
Pulse width modulation schemes are also possible with a two-position light modulator. If we define a “frame time” as the time between sequential images projected by a display, then in pulse width modulation, a pixel is set in its “on” state for a fraction of the frame time corresponding to its intensity expressed as a fraction of the maximum possible intensity.
Since contrast is achieved in digital micromirror systems by binary modulation of the micromirror position the number of possible light levels that can be shown by a digital micromirror display is set by the number of different fractional “on” times or “time slots” that can be accommodated within a frame time. Theoretically, if a large enough number of time slots could be achieved with the micromirror system any desired contrast could be achieved. However, this would require mirrors that switch arbitrarily fast.
In practice, the maximum number of time slots is set by several factors. The image must be updated at least 20–60 times per second (i.e. the frame time is approximately 1/60˜ 1/20 seconds) to prevent the human observer from noticing an artificial flicker and to prevent undesirable artifacts when an object in an image appears to move quickly across the viewing area. The mirrors also take a finite time to switch from one state to the other and stabilize in their new state. For example, in conventional display systems each micromirror takes approximately 20 microseconds to switch position. Finally, each image corresponding to a particular placement of objects in view must be repeated for red, green, and blue color channels to be presented. All of these constraints limit the number of gray levels than can be displayed.
The electromechanical control scheme for micromirrors also affects contrast. The micromirror control circuitry “must drive the micromirror from one extreme landing position to the other at very high speed as incoming video data dictates,” according to “DMD pixel mechanics simulation,” by Robert E. Meier, TI Technical Journal, p. 64–74, July–September 1998; see especially p. 66, p. 72, and FIG. 7. Also, improper timing of control pulses “can lead to undesirable contrast ratio degradation” if the micromirror either switches unintentionally or follows an undesired trajectory, ibid.
The maximum contrast achieved in a conventional micromirror display system is approximately a 1000:1 ratio between the brightest and darkest pixel presented. Human vision, however, is sensitive to brightness levels that vary by a ratio of as much as 10000:1. Therefore, methods and apparatus are desired for constructing and modulating a micromirror device such that high contrast is achieved.